Electroless plating method

ABSTRACT

A metallic film ( 2 ) made of a metal on which an electroless plating film can be deposited is formed on part of the surface of a thermoelectric semiconductor ( 8 ) which is an object to be plated, made of a constituent material to which an electroless plating can not be directly applied, and subsequently, the thermoelectric semiconductor ( 8 ) is dipped in an electroless plating bath, whereupon a conductive film ( 3 ) having a uniform thickness, made up of an electroless plating film, is formed on the entire surface of the thermoelectric semiconductor ( 8 ) containing the surface of the metallic film ( 2 ).

TECHNICAL FIELD

The invention relates to an electroless plating method for applyingelectroless plating to an object to be plated, made of a constituentmaterial to which an electroless plating can not be directly applied,and in particular, to an electroless plating method suited for forming aconductive film on end faces of metal or semiconductors, to which anelectroless plating can not be directly applied.

BACKGROUND TECHNOLOGY

As a thermoelectric device generates a voltage if the opposite endsthereof are maintained at different temperatures, the same is utilizedfor thermoelectric power generation, and conversely, if electric currentis caused to flow therethrough, an exothermic reaction occurs at one endthereof while an endothermic reaction occurs at the other end thereof.Accordingly, the same is also utilized in a cooling apparatus, and soforth, making use of an endothermic phenomenon. Because such athermoelectric device as described above is simple in construction, andhas an advantage over other electric power generators in implementationof miniaturization, and so forth, hopes run high that the same will beapplied to portable electronic equipment such as an electronic wristwatch.

The thermoelectric device is made up of a plurality of thermocouplesarranged in series, each composed of a p-type semiconductorthermoelectric material and an n-type semiconductor thermoelectricmaterial. The construction of such a common type thermoelectric deviceas above is described with reference to FIG. 19.

A thermoelectric device 10 shown in FIG. 19 has a thermoelectric deviceblock 11 wherein p-type thermoelectric semiconductors 1 and n-typethermoelectric semiconductors 1 are alternately disposed with aninsulation layer 4 made of epoxy resin, interposed therebetween,respectively. A conductive film 3 provided on an end face of therespective thermoelectric semiconductors 1, on opposite sides thereof,is connected with a wiring electrode 6 made of copper or gold, providedon substrates 7, respectively, through the intermediary of respectiveconnection layers 5, thereby rendering the thermoelectric device block11 electrically continuous with the substrates 7, and connecting therespective thermoelectric semiconductors 1 with each other in series.

Prior to connecting the thermoelectric device 10 with the substrates 7,the conductive film 3 is formed on the end face of the respectivethermoelectric semiconductors 1, on the opposite sides thereof, to beconnected with the respective wiring electrodes 6. This is necessary forthe following reasons.

The connection layers 5 are provided in order to ensure electricalcontinuity between the respective thermoelectric semiconductors 1 andthe respective wiring electrodes 6, however, if the connection layers 5are formed of solder, tin contained therein is diffused into therespective thermoelectric semiconductors 1, causing deterioration inperformance of the thermoelectric device 10. Accordingly, it isnecessary to form the conductive films 3 for elimination of such a riskand to ensure wettability of solder. Further, in the case of forming theconnection layers 5 from a conductive adhesive, it is necessary to formthe conductive films 3 having a low contact resistance against theconductive adhesive because of a large contact resistance between therespective thermoelectric semiconductors 1 and the conductive adhesive.

In the case of forming a metallic film on a thermoelectricsemiconductor, serving as a conductive film, plating is generallyadopted. In applying plating, an electroless plating method using aself-catalyzing type electroless plating bath is advantageous in termsof productivity. It is not possible, however, to apply electrolessplating to a thermoelectric semiconductor composed of an intermetalliccompound of a bismuth-tellurium base or an antimony-tellurium base.

For this reason, in the case of forming a conductive film on the surfaceof material such as a thermoelectric semiconductor to which it is notpossible to apply electroless plating, it has been a normal practice toapply electroplating thereto.

For the formation of the conductive film on the surface of thethermoelectric semiconductor by electroplating, however, electric powerneeds to be supplied to the thermoelectric semiconductor, which hascaused a problem in that the thickness of a plating film formed becomesthinner according as a distance from the point of power supply increasesdue to a voltage drop caused by a resistance value of the thermoelectricsemiconductor. This has resulted in fluctuation in the thickness of theconductive film made up of the plating film, thereby impairing an effectof preventing diffusion of tin contained in solder, and adverselyaffecting wettability of solder.

In JP11-186619, a method of applying electroless plating by providing athermoelectric semiconductor with a catalyst, such as platinum,palladium, and so forth, is disclosed as a method of forming aconductive film on a constituent material to which it is not possible toapply electroless plating.

This method, however, is a method whereby electroless plating isimplemented by providing a catalyst as seed crystals, and is a methodgenerally adopted for forming a conductive film on plastics. With themethod described, there is eliminated the abovementioned problem ofuneven thickness of the plating film formed by electroplating, but thefollowing problem has been encountered.

That is, with this method, since adsorption of the catalyst to serve asthe seed crystals occurs to parts other than the thermoelectricsemiconductor, selectivity on regions where the conductive films are tobe formed will be lost upon dipping the thermoelectric semiconductor inan electroless plating bath, causing a problem that the formation of theconductive films occurs to unnecessary regions as well, for example, onthe surface of insulators.

Thus, there have so far existed not only a problem that it has not beenpossible to form the conductive films on the surface of a constituentmaterial to which it is not possible to apply electroless plating, butalso a problem that selectivity on the regions where the conductivefilms are to be formed has been lost even if the conductive films havebeen formed by electroless plating.

In particular, the thermoelectric device comprises thermoelectricsemiconductors which are very small in size, and has sometimes aminuscule structure wherein the thermoelectric semiconductors aredisposed at an interval between the adjacent thermoelectricsemiconductors, in a range of several to several tens of μm. The moreminuscule the structure of the thermoelectric device, the more difficultit becomes to form the conductive films selectively only on thethermoelectric semiconductors. It is therefore a major problem in thefabrication of the thermoelectric device to selectively form theconductive films by electroless plating.

The invention has been developed to solve those problems, and an objectof the invention is to provide an electroless plating method wherebyconductive films can be formed even on the surface of a constituentmaterial to which it is not possible to apply electroless plating, andfurther, to selectively form the conductive films uniform in thicknesson end faces of respective thermoelectric semiconductors formed of aconstituent material to the surface of which it is not possible to applyelectroless plating, thereby enhancing productivity and reliability of athermoelectric device as fabricated.

DISCLOSURE OF THE INVENTION

An electroless plating method according to the invention comprises thesteps of forming a metallic film made of a metal on which an electrolessplating film can be deposited on part of the surface of an object to beplated, or causing the metal to be in contact with part of the surfaceof the object to be plated, made of a constituent material to which anelectroless plating can not be applied, and dipping the object to beplated having the metallic film formed thereon or having the metal incontact therewith in an electroless plating bath, and forming anelectroless plating film on the surface of the object to be plated,without the metallic film formed thereon and the metal in contacttherewith.

Further, the electroless plating method according to the invention maycomprise the steps of forming a metallic film made of a metal on whichan electroless plating film can be deposited on part of the surface ofan object to be plated, or causing the metal to be in contact with partof the surface of the object to be plated, made of a constituentmaterial to which an electroless plating can not be applied, dipping theobject to be plated having the metallic film formed thereon or havingthe metal in contact therewith in an electroless plating bath, andforming an electroless plating film on the entire surface of the objectto be plated, containing the metallic film or the metal, removing themetallic film or the metal, and portions of the electroless platingfilm, covering up the metallic film or the metal, from the object to beplated, and dipping again the object to be plated subjected to the stepsdescribed above in the electroless plating bath.

With any of the electroless plating methods described above, the objectto be plated may be made of plural kinds of constituent materials or maybe a thermoelectric semiconductor.

Further, with any of the electroless plating methods described above,the electroless plating film may be formed so as to have a dual-layerstructure comprised of not less than two metallic films.

The electroless plating method according to the invention, applied tothe fabrication of a thermoelectric device, may comprise the followingrespective steps:

-   -   (1) the step of forming a metallic film made of a metal on which        an electroless plating film can be deposited on one of end faces        of a thermoelectric device block formed integrally with a        plurality of thermoelectric semiconductors, disposed with an        insulation layer interposed therebetween, respectively;    -   (2) the step of dipping the thermoelectric device block having        the metallic film formed thereon in an electroless plating bath,        and forming an electroless plating film on the metallic film and        the other end face of the respective thermoelectric        semiconductors, on the side thereof, opposite from the end face        on which the metallic film is formed;    -   (3) the step of removing the metallic film and a portion of the        electroless plating film, covering up the metallic film; and    -   (4) the step of dipping again the thermoelectric device block        subjected to the steps described above in the electroless        plating bath, and forming an electroless plating film on the end        face of the respective thermoelectric semiconductors from which        the metallic film is removed.

With the electroless plating methods described above, the followingsteps (5) to (8) may be substituted for the abovementioned steps (1) to(4):

-   -   (5) the step of causing a metal on which an electroless plating        film can be deposited to be in contact with a part of at least        one of end faces of respective thermoelectric semiconductors of        a thermoelectric device block formed integrally with a plurality        of thermoelectric semiconductors, disposed with an insulation        layer interposed therebetween, respectively;    -   (6) the step of dipping the thermoelectric device block having        the metal in contact therewith in an electroless plating bath,        and forming an electroless plating film on the entire surface of        the respective thermoelectric semiconductors, except the part        thereof, in contact with the metal,    -   (7) the step of separating the metal in contact with the        respective thermoelectric semiconductors therefrom; and    -   (8) the step of dipping again the thermoelectric device block        subjected to the steps described above in the electroless        plating bath, and forming an electroless plating film on the        part of the end faces of the respective thermoelectric        semiconductors, in contact with the metal.

Further, with the electroless plating methods described above, thefollowing steps (9) and (10) may be substituted for the abovementionedsteps (1) to (8):

-   -   (9) the step of forming a metallic film made of a metal on which        an electroless plating film can be deposited on an end face of        respective insulation layers disposed on the side of one of end        faces of a thermoelectric device block formed integrally with a        plurality of thermoelectric semiconductors, disposed with the        respective insulation layers interposed therebetween, such that        the metallic film spans the respective insulation layers and a        portion of respective end faces of both the thermoelectric        semiconductors adjacent to each other across the respective        insulation layers alternately disposed; and    -   (10) the step of dipping the thermoelectric device block having        the metallic film formed thereon in an electroless plating bath,        and forming an electroless plating film on the metallic film and        both end faces of the respective thermoelectric semiconductors        with the metallic film formed on the portion of the end face        thereof.

Still further, with the electroless plating methods described above, thefollowing steps (11) and (12) may be substituted for the abovementionedsteps (1) to (8):

-   -   (11) the step of forming a metallic film made of a metal on        which an electroless plating film can be deposited on either an        end face or the other end face of respective insulation layers,        alternately, on the sides of both end faces of a thermoelectric        device block formed integrally with a plurality of        thermoelectric semiconductors, disposed with the respective        insulation layers interposed therebetween, such that the        metallic film spans the respective insulation layers and a        portion of respective end faces of both the thermoelectric        semiconductors adjacent to each other across the respective        insulation layers; and    -   (12) the step of dipping the thermoelectric device block having        the metallic film formed thereon in an electroless plating bath,        and forming an electroless plating film on the metallic film and        both end faces of the respective thermoelectric semiconductors        with the metallic film formed on the portion of the end face and        the other end face thereof.

Yet further, with any of the electroless plating methods comprising theabovementioned steps (1) to (12), use may be made of the thermoelectricdevice block provided with an exposed outer sidewall face of respectivethermoelectric semiconductors positioned at opposite ends in thedirection along which the respective thermoelectric semiconductors arearranged, and an electroless plating film may be also formed on theexposed outer sidewall faces as well in the step of forming theelectroless plating film.

Further, in the case of applying the electroless plating methodaccording to the invention to the fabrication of a thermoelectricdevice, the electroless plating method preferably comprises the step ofrendering the end face of the thermoelectric device block into a roughsurface prior to the step of forming the electroless plating film on thethermoelectric device block.

Still further, the electroless plating method preferably comprises thestep of cleaning the thermoelectric device block before or after thestep of forming the electroless plating film on the thermoelectricdevice block.

And further, the present invention provides an electroless platingmethod comprising the steps of preparing an object to be plated,comprised of metal or semiconductors, to which an electroless platingcan not be applied, and insulators, and forming a metallic film made ofa metal on which an electroless plating film can be deposited on part ofthe surface of the object to be plated, or causing the metal to be incontact with part of the surface of the object to be plated, and dippingthe object to be plated having the metallic film formed thereon orhaving the metal in contact therewith in an electroless plating bath,and forming an electroless plating film on the entire surface of theobject to be plated, except for the insulators.

As the constituent material to which an electroless plating can not beapplied, use can be made of a metal or a semiconductor, to which anelectroless plating can not be applied.

As the metal on which the electroless plating film can be deposited, usecan be made of palladium, platinum or nickel.

An insulating resin is preferably used for the insulators or theinsulation layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a state wherein a metallic film madeof a metal on which an electroless plating film can be deposited isformed on part of the surface of a thermoelectric semiconductor by theinvention;

FIG. 2 is a sectional view showing a state wherein a conductive filmaccording to an electroless plating film is formed on the entire surfaceof the thermoelectric semiconductor and the metallic film;

FIG. 3 is a sectional view schematically showing a thermoelectric deviceblock to which electroless plating is applied by the invention;

FIGS. 4 to 8 are sectional views sequentially showing respective stepsof applying electroless plating to the thermoelectric device blockaccording to a first embodiment of the invention;

FIGS. 9 to 11 are sectional views sequentially showing respective stepsof applying electroless plating to the thermoelectric device blockaccording to a second embodiment of the invention;

FIGS. 12 and 13 are sectional views sequentially showing respectivesteps of applying electroless plating to the thermoelectric device blockaccording to a third embodiment of the invention;

FIGS. 14 to 16 are sectional views sequentially showing respective stepsof applying electroless plating to the thermoelectric device blockaccording to a fourth embodiment of the invention;

FIG. 17 is a sectional view showing a state wherein probes are caused tobe in contact with the thermoelectric device block in applyingelectroless plating to the thermoelectric device block according to thefirst embodiment of the invention;

FIG. 18 is a sectional view showing state wherein the metallic film isformed on the another thermoelectric device block in applyingelectroless plating to the thermoelectric device block according to thefourth embodiment of the invention; and

FIG. 19 is a sectional view schematically showing the construction of acommon type thermoelectric device.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of an electroless plating method according to theinvention are described hereinafter with reference to the accompanyingdrawings. First, the basic embodiment of the electroless plating methodaccording to the invention is described with reference to FIGS. 1 and 2.

Basic Embodiment; FIGS. 1 and 2

FIG. 1 is a sectional view showing a state wherein a metallic film madeof a metal on which an electroless plating film can be deposited isformed on part of the surface of a thermoelectric semiconductor which isan example of an object to be plated, made of a constituent material towhich an electroless plating can not be directly applied.

A thermoelectric semiconductor 8 is formed in a block shape, and isgenerally made of an intermetallic compound selected from the groupconsisting of bismuth-tellurium based compound, antimony-tellurium basedcompound, bismuth-tellurium-antimony based compound,bismuth-tellurium-selenium based compound, and so forth, but the samemay be formed of an intermetallic compound selected from the groupconsisting of lead-germanium based compound, silicon-germanium compound,and so forth, although not limited particularly to those mentionedabove.

In applying the electroless plating method according to the invention, ametallic film 2 made of a metal on which an electroless plating film canbe deposited is first formed on part of the surface of thethermoelectric semiconductor 8 by the vacuum deposition method, thesputtering method, or so forth as shown in FIG. 1. The metallic film 2formed at this point in time may be made of any metal causing depositionof a metal in an electroless plating bath. For example, in the case ofexecuting electroless nickel plating, use is made of a metal such aspalladium, platinum, nickel or so froth. Further, the metallic film 2may be formed by disposing a conductive resin such as a conductivepaste, composed of particles of a metal on which an electroless platingfilm can be deposited and insulating resin, by the printing method, andso forth, besides by the vacuum deposition method, or the sputteringmethod.

Thereafter, the thermoelectric semiconductor 8 with the metallic film 2formed thereon is dipped in an electroless plating bath (not shown),whereupon an electroless plating film is first deposited on the surfaceof the metallic film 2. Because the metallic film 2 is in contact withthe thermoelectric semiconductor 8 at this point in time, the potentialof the thermoelectric semiconductor 8 relative to the electrolessplating bath (a condition for effecting transfer of electrons with themetal in electroless plating bath) undergoes a change, thereby allowingthe electroless plating film to be deposited on the thermoelectricsemiconductor 8. Accordingly, the electroless plating film depositedfrom the metallic film 2 spreads out to the thermoelectric semiconductor8, so that a conductive film 3 according to the electroless plating filmhaving a uniform thickness is formed on the entire surface of thethermoelectric semiconductor 8 and the metallic film 2 as shown in FIG.2.

In the case of a constituent material to which the electroless platingcan not be directly applied being the aforementioned thermoelectricsemiconductor, the conductive film 3 is preferably formed of nickel (Ni)highly effective in preventing diffusion of tin, copper, and so forthinto the thermoelectric semiconductor although the constituent materialthereof is not limited particularly to nickel.

Further, the conductive film 3 may be formed by depositing not less thantwo kinds of metallic films, one on top of another. For example, theconductive film 3 may be formed by depositing a metallic film made ofgold (Au) or copper (Cu) on a metallic film made of nickel so as to havea dual-layer structure. By so doing, it becomes possible to preventoccurrence of cracks otherwise occurring to the metallic film formed ofnickel when subjected to stress or thermal stress owing to extensibilityof gold (Au) or copper (Cu), thereby enhancing reliability of athermoelectric device.

With the method described above, it becomes possible to form aconductive film uniform in thickness by electroless plating even on athermoelectric semiconductor made of a constituent material on which ithas been considered that the conductive film can not be depositeddirectly, so that productivity of a thermoelectric device employingthermoelectric semiconductors can be improved.

An object to be plated which this method is applicable to is not limitedto the thermoelectric semiconductor. It becomes possible to form aconductive film made of a metal having a high conductivity by theelectroless plating method even on a metal, cadmium, tungsten, zinc,tin, lead, bismuth, antimony and so forth, to which it has been regardedimpossible to apply electroless plating.

Further, instead of forming the metallic film made of the metal on whichthe electroless plating film can be deposited on part of the surface ofthe object to be plated as described above, the metal on which theelectroless plating film can be deposited may be brought into contactwith an object to be plated, such as a thermoelectric semiconductor, andso forth, and with such a contact condition maintained by use of a toolsuch as a clip, the object to be plated may be dipped in an electrolessplating bath. With such a method as well, it is possible to obtain thesame effect of forming a conductive film uniform in thickness on theentire surface of the object to be plated. In this case, a clip made ofa metal on which an electroless plating film can be deposited may bebrought into direct contact with the object to be plated. Further, notthe whole, but only part of a clip, coming into contact with the objectto be plate, may be formed of a metal on which an electroless platingfilm can be deposited.

Furthermore, after removing the metallic film 2, and a portion 3 a ofthe conductive film 3, covering the metallic film 2, shown in FIG. 2,the thermoelectric semiconductor 8 may be dipped again in theelectroless plating bath. By so doing, the conductive film 3 can beformed on the entire surface of the thermoelectric semiconductor 8.

Embodiments of the electroless plating method according to the inventionfor applying electroless plating to a thermoelectric device block willbe described in detail hereinafter with reference to FIGS. 3 to 18. Inthese figures, parts corresponding to those in FIG. 19 are denoted bylike reference numerals.

First Embodiment: FIGS. 3 to 8, and FIG. 17

First, a first embodiment of the invention is described with referenceto FIGS. 3 to 8, and FIG. 17. This method for applying electrolessplating to a thermoelectric device block represents an application ofthe electroless plating method according to the invention as describedhereinbefore.

FIG. 3 is a sectional view of a thermoelectric device block 11 which isan object to be plated. With the thermoelectric device block 11, p-typeand n-type thermoelectric semiconductors 1, each in a bar-like shape,are alternately disposed at an interval in a range of about 5 to 80 μmwith an insulation layer 4 made of epoxy resin, interposed therebetween,respectively, and the respective thermoelectric semiconductors 1adjacent to each other are isolated by the insulation layer 4.

As with the thermoelectric semiconductors 8 described in the foregoing,the thermoelectric semiconductors 1 are made of an intermetalliccompound in common use, selected from the group consisting ofbismuth-tellurium based compound, antimony-tellurium based compound,bismuth-tellurium-antimony based compound, bismuth-tellurium-seleniumbased compound, or an intermetallic compound selected from the groupconsisting of lead-germanium based compound, silicon-germanium basedcompound, and so forth, although not limited to those mentioned above.

The thermoelectric device block 11 is formed as follows. First, athermoelectric semiconductor block (not shown) worked into a comb-toothlike shape, with a plurality of grooves provided at a predeterminedpitch, is prepared for a p-type and an n-type thermoelectricsemiconductors, respectively. Then, these thermoelectric semiconductorblocks are combined with each other such that partition walls ofrespective grooves of the thermoelectric semiconductor block are fittedinto respective grooves of the other thermoelectric semiconductor block,epoxy resin is poured into a gap therebetween, and subsequently, theepoxy resin as poured is cured by applying heat treatment thereto,thereby forming a united block. Thereafter, unnecessary parts of theunited block are removed by grinding, whereupon the thermoelectricdevice block 11 can be obtained.

Subsequently, by the vacuum deposition method, the sputtering method orso forth, a metallic film 2 is formed on the entire surface of one endface 11 a of end faces 11 a, 11 b of the thermoelectric device block 11,containing end faces 1 a, 1 b of the respective thermoelectricsemiconductors 1, respectively, as shown in FIG. 4. The metallic film 2is a film formed of a metal on which an electroless plating film can bedeposited, that is, a metal to which deposition reaction of a metal inan electroless plating bath occurs. For example, in the case ofelectroless nickel plating, the metallic film 2 is formed of a metalselected from the group consisting of palladium, platinum, nickel, andso forth. Further, instead of forming the metallic film 2 by the vacuumdeposition method or the sputtering method, the same may be formed bydisposing a conductive resin such as a conductive paste, composed ofparticles of a metal on which an electroless plating film can bedeposited and insulating resin, by the printing method, and so forth.

Subsequently, the thermoelectric device block 11 with the metallic film2 formed thereon is dipped in an electroless plating bath, whereupondeposition reaction of an electroless plating film occurs to the surfaceof the metallic film 2, as shown in FIG. 5, and simultaneously, thepotential of the respective thermoelectric semiconductor 1 relative tothe electroless plating bath (a condition for effecting transfer ofelectrons with the metal in electroless plating bath) undergoes achange, so that deposition reaction of the electroless plating filmoccurs to the end face 1 b as well, on the side of the respectivethermoelectric semiconductor 1, where the metallic film 2 is not formed.Thus, a conductive film 3 which is the electroless plating film can beformed directly only on the end face 1 b of the respectivethermoelectric semiconductors 1.

Then, the metallic film 2 and a portion of the conductive film 3, formedon top of the metallic film 2 so as to cover up the same, is removed byetching, as shown in FIG. 6, and thereafter, the thermoelectric deviceblock 11 is dipped again in the electroless plating bath, whereupon aconductive film 3 can be formed selectively only on the end face la ofthe respective thermoelectric semiconductors 1, exposed by removing themetallic film 2 by means of etching, as shown in FIG. 7. With such amethod as described above, since the conductive film 3 will not beformed on unnecessary parts such as the insulation layers 4, electricalinsulation between the respective thermoelectric semiconductors 1 can beensured, so that a highly reliable thermoelectric device provided withthe conductive film 3 formed only on both the end faces 1 a, 1 b of therespective thermoelectric semiconductors 1 can be obtained.

Further, instead of forming the conductive films 3 on the thermoelectricdevice block 11 as described in the foregoing, the following process maybe adopted. First, a probe 14 made of a metal on which an electrolessplating film can be deposited, in the shape of a needle as shown in FIG.17, is caused to be in contact with a part of the end face 1 b of therespective thermoelectric semiconductors 1, or a plate (not shown) madeof a metal on which an electroless plating film can be deposited, formedin a shape corresponding to the end face 11 a (11 b) of thethermoelectric device block 11, is caused to be in contact with the endface 1 b of the respective thermoelectric semiconductors 1. Thereafter,the thermoelectric device block 11 with the probes 14 in contacttherewith is dipped in an electroless plating bath, thereby causing anelectroless plating film to be deposited on the entire surface of therespective thermoelectric semiconductors 1, except for a part thereof,in contact with the probe 14. Subsequently, after separating the probes14 from the respective thermoelectric semiconductors 1, thethermoelectric device block 11 is dipped again in the electrolessplating bath, thereby causing an electroless plating film to bedeposited on the part of the surface of the respective thermoelectricsemiconductors 1, in contact with the probe 14. In this way, it is alsopossible to form the conductive film 3 only on both the end faces 1 a, 1b of the respective thermoelectric semiconductors 1.

At the time when the previously described etching is performed, aphotoresist (not shown) is applied to the entire surface of thethermoelectric device block 11, on the side of the end face 11 bthereof, shown in FIG. 5. The reason for this is because the conductivefilm 3 already formed selectively on the end face 1 b, on one side ofthe respective thermoelectric semiconductors 1, needs to be protected bythe photoresist while the metallic film 2 and the conductive film 3,formed on the side of the end face 11 a of the thermoelectric deviceblock 11, need to be removed with reliability. In this connection, thereis available a method of removing the metallic film 2 and the conductivefilm 3 that are unnecessary by grinding besides the etching.

Nickel is preferably used for the conductive films 3 formed byelectroless plating in that nickel is highly effective in preventingdiffusion of tin, copper and so forth into the respective thermoelectricsemiconductors 1, however, a metal for use in the conductive films 3 isnot limited particularly to nickel. Further, the conductive films 3 maybe formed by depositing not less than two kinds of metallic films, oneon top of another. For example, the conductive films 3 may be formed bydepositing a metallic film made of gold (Au) or copper (Cu) on ametallic film made of nickel so as to have a dual-layer structure. By sodoing, it becomes possible to prevent occurrence of cracks otherwiseoccurring to the metallic film formed of nickel when subjected to stressor thermal stress owing to extensibility of gold (Au) or copper (Cu),thereby enhancing reliability of a thermoelectric device.

Subsequently, as shown in FIG. 8, connection layers 9 made of aconnecting material such as a conductive adhesive or a solder paste areformed by the printing method on the thermoelectric device block 11provided with the conductive film 3 formed on the end faces 1 a, 1 b,respectively, on opposite sides of the respective thermoelectricsemiconductors 1, shown in FIG. 7. The p-type thermoelectricsemiconductors 1 and the n-type thermoelectric semiconductors 1 arealternately connected with each other, respectively, via the respectiveconnection layers 9, and upon applying heat treatment thereto, therespective thermoelectric semiconductors 1 are electrically connected inseries, thereby completing a thermoelectric device 20.

For obtaining the thermoelectric device 20 by connecting the respectivethermoelectric semiconductors 1 in series, the method shown in FIG. 19may be adopted. That is, the substrates 7, each provided with the wiringelectrode 6 made of copper or gold, formed thereon, are prepared, and byconnecting the wiring electrodes 6 with the conductive films 3,respectively, through the intermediary of the respective connectionlayers 5 formed of solder, a conductive adhesive, an anisotropicconductive adhesive, or so forth, the respective thermoelectricsemiconductors 1 may be connected with each other in series.

Second Embodiment: FIG. 3 and FIGS. 9 to 11

Subsequently, a second embodiment of a method for applying electrolessplating according to the invention to a thermoelectric device block isdescribed hereinafter with reference to FIG. 3 and FIGS. 9 to 11.

With this embodiment, use is made of the thermoelectric device block 11shown in FIG. 3 as with the case of the first embodiment, and for otherparts such as metallic films, conductive films, an electroless platingbath, and so froth, use is also made of the same constituent materialsas those used for the corresponding parts in the first embodiment.

First, metallic films 2 on which an electroless plating film can bedeposited are formed on an end face 11 a of the thermoelectric deviceblock 11 shown in FIG. 3, on one side thereof, by the vacuum depositionmethod, the sputtering method or so forth. As shown in FIG. 9, therespective metallic film 2 are formed with the use of a metal mask, andso forth, selectively only on a portion of an end face 1 a of respectivethermoelectric semiconductors 1, on one side thereof, necessary forconnecting adjacent p-type and n-type thermoelectric semiconductors 1together with an insulation layer 4 interposed therebetween. Morespecifically, each of the metallic films 2 is formed on an end face 4 aof the respective insulation layers 4 alternately disposed, and on aportion of the end face 1 a of the respective thermoelectricsemiconductors 1, on both sides of the end face 4 a, so as to span boththe thermoelectric semiconductors 1 adjacent to each other across theinsulation layer 4 on the end face 11 a of the thermoelectric deviceblock 11, such that the insulation layer 4 with the metallic film 2formed thereon and the insulation layer 4 without the metallic film 2formed thereon are disposed in an alternating sequence on the end face11 a.

Subsequently, the thermoelectric device block 11 provided with themetallic films 2 formed as described above is dipped in an electrolessplating bath, whereupon deposition reaction of an electroless platingfilm occurs to the surface of the respective metallic films 2, andsimultaneously, deposition reaction of an electroless plating film alsooccurs to the end face la of the respective thermoelectricsemiconductors 1 with the metallic film 2 formed on (in contact with) aportion thereof, and to an end face 1 b, opposite from the end face 1 a,as shown in FIG. 10. Thus, a conductive film 3 can be formed only on theend face 1 a of the respective thermoelectric semiconductors 1,containing the metallic film 2, and on the end face 1 b, opposite fromthe end face 1 a.

Thereafter, as shown by the phantom lines in FIG. 10, a connection layermade of a connecting material such as a conductive adhesive or a solderpaste is formed by the printing method on the end face 1 b of therespective thermoelectric semiconductors 1, with the conductive film 3selectively formed thereon, thereby alternately connecting therespective p-type thermoelectric semiconductors 1 with the respectiven-type thermoelectric semiconductors 1. Upon applying heat treatmentthereto, there is obtained a thermoelectric device wherein therespective thermoelectric semiconductors 1 are electrically connectedtogether in series.

For obtaining the thermoelectric device by connecting the respectivethermoelectric semiconductors 1 in series, a substrate 7 with a wiringelectrode 6 made of copper or gold, formed thereon, may be used, and byelectrically connecting the respective conductive films 3 on the endface 11 a side of the thermoelectric device block 11 with the wiringelectrode 6 on the substrate 7 through the intermediary of respectiveconnection layers 5 formed of solder, a conductive adhesive, ananisotropic conductive adhesive, or so forth, as shown in FIG. 11, therespective thermoelectric semiconductors 1 may be connected with eachother in series, thereby completing a thermoelectric device 21.

In contrast with the first embodiment as previously described, accordingto the second embodiment, a processing step of removing the metallicfilm 2 formed on the end face 11 a, on one side of the thermoelectricdevice block 11, is not required, thereby enabling a process up to thecompletion of the thermoelectric device to be shortened. Accordingly,productivity in fabrication of the thermoelectric device can beimproved.

Third Embodiment: FIG. 3 and FIGS. 12 and 13

Subsequently, a third embodiment of a method for applying electrolessplating according to the invention to a thermoelectric device block isdescribed hereinafter with reference to FIG. 3 and FIGS. 12 and 13.

With this embodiment, use is made of the thermoelectric device block 11shown in FIG. 3 as with the case of the first embodiment, and for otherparts such as metallic films, conductive films, an electroless platingbath, and so froth, use is also made of the same constituent materialsas those used for the corresponding parts in the first embodiment.

First, metallic films 2 on which an electroless plating film can bedeposited are formed on end faces 11 a, 11 b of the thermoelectricdevice block 11 shown in FIG. 3, on opposite sides thereof, by thevacuum deposition method, the sputtering method or so forth, as shown inFIG. 12. With the use of a metal mask, and so forth, each of themetallic films 2 is formed selectively only on end face 4 a, and endface 4 b of respective insulation layers 4, in an alternate andstaggered sequence, that is, on those where the metallic film 2 isrequired for connecting respective p-type thermoelectric semiconductors1 and respective n-type thermoelectric semiconductors 1, disposed onopposite sides of the respective insulation layers 4, with each other,thereby connecting the respective thermoelectric semiconductors 1 inseries. More specifically, each of the metallic films 2 is formed so asto span a part of the end faces 1 a or the end faces 1 b of the adjacentthermoelectric semiconductors 1 with the respective insulation layers 4interposed therebetween, and also, on the end face 4 a and the other endface 4 b of the respective insulation layers 4, alternately.

Subsequently, the thermoelectric device block 11 provided with themetallic films 2 is dipped in an electroless plating bath, whereupondeposition reaction of an electroless plating film occurs to the surfaceof the respective metallic films 2, as shown in FIG. 13, andsimultaneously, deposition reaction of an electroless plating filmoccurs to the end face 1 a or 1 b of the thermoelectric semiconductors 1without the metallic film 2 opposite from the end face 1 a or 1 b withthe metallic film 2 formed on (in contact with) part thereof. Thus, aconductive film 3 can be formed only on the end face 1 a and 1 b of therespective thermoelectric semiconductors 1, and the respective metallicfilms 2

Because the respective thermoelectric semiconductors 1 of thethermoelectric device block 11 are connected in series via therespective conductive films 3, a thermoelectric device 22 wherein therespective thermoelectric semiconductors 1 are connected in series canbe obtained without taking processing steps of alternately connectingthe adjacent thermoelectric semiconductors 1 by forming the connectionlayers and using the substrates as with the case of the first and secondembodiments. Accordingly, in comparison with the first and secondembodiments, a process up to the completion of the thermoelectric devicecan be shortened, thereby improving productivity in fabrication of thethermoelectric device.

Fourth Embodiment: FIGS. 14 to 16 and FIG. 18

Subsequently, a fourth embodiment of a method for applying electrolessplating according to the invention to a thermoelectric device block isdescribed hereinafter with reference to FIGS. 14 to 16 and FIG. 18.

In contrast with the first to third embodiments, with this embodiment,use is made of a thermoelectric device block 15 wherein the outersidewall face of thermoelectric semiconductors 1 among respectivethermoelectric semiconductors 1, positioned at opposite ends in thedirection of arrangement thereof, is not coated with an insulation layer4 so as to be exposed as shown FIG. 14, however, for other parts such asmetallic films, conductive films, an electroless plating bath, and sofroth, use is made of the same constituent materials as those used forthe corresponding parts in the first embodiment.

With this embodiment, a metallic film 2 is first formed on an end face 1a or 1 b of the respective thermoelectric semiconductors 1 in the sameway as in any of the first to third embodiments. In the case of formingthe metallic films 2 in the same way as in the third embodiment, each ofthe metallic films 2 on which an electroless plating film can bedeposited is formed on an end face 4 a and the other end face 4 b of therespective insulation layers 4 of the thermoelectric device block 15alternately as to span a part of the end faces 1 a or the end faces 1 bof the adjacent thermoelectric semiconductors 1 with the respectiveinsulation layers 4 interposed therebetween, as shown in FIG. 14. In thecase of forming the metallic film 2 in the same way as in the firstembodiment, the metallic film 2 are formed as shown in FIG. 4. In thecase of forming the metallic films 2 in the same way as in the secondembodiment, the metallic films 2 are formed as shown in FIG. 18.

Subsequently, this thermoelectric device block 15 provided with themetallic films 2 is dipped in an electroless plating bath, whereupondeposition reaction of an electroless plating film occurs to the surfaceof the respective metallic films 2, and simultaneously, depositionreaction of an electroless plating film occurs to the end face 1 a and 1b of the thermoelectric semiconductors 1 with the metallic film 2 formedon (in contact with) part thereof, and also to the outer sidewall faceas exposed of the respective thermoelectric semiconductors 1 positionedon the outermost sides the thermoelectric device block 15 (at oppositeends in the direction along which the respective thermoelectricsemiconductors 1 are arranged). Thus, a conductive film 3 can be formedon the respective metallic films 2, the end face 1 a and 1 b of therespective thermoelectric semiconductors 1, on opposite sides thereof,and the outer sidewall face as exposed of the respective thermoelectricsemiconductors 1 positioned at the opposite ends except for an end faceof respective insulation layers 4 without the metallic film 2 formedthereon, thereby enabling the respective thermoelectric semiconductors 1to be connected in series.

Thereafter, after forming connection layers 19 made of a connectingmaterial such as a conductive adhesive, solder or so forth, thethermoelectric device block 15 with the conductive films 3 formedthereon is mounted on a substrate 7 with a wiring electrode 6 formedthereon as shown in FIG. 16. The respective conductive films 3 of thethermoelectric device block 15 are thereby electrically connected withthe wiring electrode 6, thus obtaining a thermoelectric device 23. Inthis case, with the thermoelectric device block 15 (FIG. 15), since theconductive film 3 is also formed on the outer sidewall face as exposedof the respective thermoelectric semiconductors 1 positioned at theopposite ends in the direction of arrangement thereof, a contact area ofthe connection layers 19 can be enlarged. As a result, connection of thewiring electrode 6 with the respective conductive films 3 can beimplemented with ease, further enabling a connection condition to beensured.

With any of the first to fourth embodiments described hereinbefore, thesurface of the thermoelectric device block, an object to be plated, onwhich the metallic films 2 or the conductive films 3 are formed, ispreferably kept in a rough condition by various methods such as etching,sandblasting, grinding or so forth. Such a practice is more effective inimprovement in reliability of the thermoelectric device because it willimprove an adhesive property of the conductive films, thereby formingmore reliable conductive films.

Further, with any of the first to fourth embodiments describedhereinbefore, it is preferable to take a cleaning process step of alkalidegreasing, ultrasonic cleaning, running water cleaning or so forthbetween respective process steps. Such a practice is effective infurther improvement in reliability of the thermoelectric device becauseit can further enhance adhesion between the respective conductive films3 and the respective thermoelectric semiconductors 1.

INDUSTRIAL APPLICABILITY

With an electroless plating method according the invention, it becomespossible to form a conductive film formed of a metal having a highconductivity by applying electroless plating directly even to aconstituent material on which it has been considered that the conductivefilm by electroless plating can not be deposited directly.

Even with a thermoelectric device block wherein insulation layers andthermoelectric semiconductors are alternately disposed at a minusculeinterval in a range of several to several tens of μm, it becomespossible to selectively form the conductive film uniform in thicknessonly on both end faces of the respective thermoelectric semiconductorsby applying the invention to a method of fabricating a thermoelectricdevice. Accordingly, the conductive films having an object of providingthe thermoelectric device with the connection layer for the respectivethermoelectric semiconductors, and having an effect of preventingdiffusion of tin, copper and so forth into the respective thermoelectricsemiconductors can be easily formed to a uniform thickness on both theend faces of the respective thermoelectric semiconductors, so thatproductivity and reliability of the thermoelectric device can beimproved.

1. An electroless plating method comprising the steps of: formingmetallic film made of a metal on which an electroless plating film canbe deposited on part of the surface of an object to be plated, orcausing the metal to be in contact with part of the surface of theobject to be plated, made of a constituent material to which anelectroless plating can not be applied; and dipping the object to beplated having said metallic film formed thereon or having said metal incontact therewith in an electroless plating bath, and forming anelectroless plating film on the surface of the object to be plated,without the metallic film formed thereon and the metal in contacttherewith.
 2. An electroless plating method comprising the steps of:forming a metallic film made of a metal on which an electroless platingfilm can be deposited on part of the surface of an object to be plated,or causing the metal to be in contact with part of the surface of theobject to be plated, made of a constituent material to which anelectroless plating can not be applied; dipping the object to be platedhaving said metallic film formed thereon or having said metal in contacttherewith in an electroless plating bath, and forming an electrolessplating film on the entire surface of the object to be plated,containing said metallic film or the metal; removing said metallic filmor the metal, and portions of the electroless plating film, covering upthe metallic film or the metal, from said object to be plated; anddipping again the object to be plated subjected to the steps describedabove in the electroless plating bath.
 3. An electroless plating methodcomprising the steps of: forming a metallic film made of a metal onwhich an electroless plating film can be deposited on part of thesurface of an object to be plated, or causing the metal to be in contactwith part of the surface of the object to be plated, made of aconstituent material to which an electroless plating can not be applied;and dipping the object to be plated having the metallic film formedthereon or having the metal in contact therewith in an electrolessplating bath, forming an electroless plating film on the surface of theobject to be plated, without the metallic film formed thereon and themetal in contact therewith, wherein said object to be plated is made ofplural kinds of constituent materials.
 4. The electroless plating methodof claim 1, wherein said object to be plated is a thermoelectricsemiconductor.
 5. An electroless plating method according to claim 2,wherein said electroless plating film is formed so as to have adual-layer structure comprised of not less than two metallic films. 6.An electroless plating method comprising the steps of: forming ametallic film made of a metal on which an electroless plating film canbe deposited on one of end faces of a thermoelectric device block formedintegrally with a plurality of thermoelectric semiconductors, disposedwith an insulation layer interposed therebetween, respectively; dippingsaid thermoelectric device block having the metallic film formed thereonin an electroless plating bath, and forming an electroless plating filmon said metallic film and the other end face of the respectivethermoelectric semiconductors, on the side thereof, opposite from theend face on which the metallic film is formed; removing said metallicfilm and a portion of the electroless plating film covering up themetallic film; and dipping again the thermoelectric device blocksubjected to the steps described above in the electroless plating bath,and forming an electroless plating film on the end face of therespective thermoelectric semiconductors from which the metallic film isremoved.
 7. An electroless plating method comprising the steps of:causing a metal on which an electroless plating film can be deposited tobe in contact with a part of at least one of end faces of respectivethermoelectric semiconductors of a thermoelectric device block formedintegrally with a plurality of thermoelectric semiconductors, disposedwith an insulation layer interposed therebetween, respectively; dippingthe thermoelectric device block having said metal in contact therewithin an electroless plating bath, and forming an electroless plating filmon the entire surface of the respective thermoelectric semiconductors,except the part thereof, in contact with said metal, separating themetal in contact with the respective thermoelectric semiconductorstherefrom; and dipping again the thermoelectric device block subjectedto the steps described above in the electroless plating bath, andforming an electroless plating film on the part of the end faces of therespective thermoelectric semiconductors, in contact with said metal. 8.An electroless plating method comprising the steps of: forming ametallic film made of a metal on which an electroless plating film canbe deposited on an end face of respective insulation layers disposed onthe side of one of end faces of a thermoelectric device block formedintegrally with a plurality of thermoelectric semiconductors, disposedwith the respective insulation layers interposed therebetween, such thatthe metallic film spans said respective insulation layers and a portionof respective end faces of both the thermoelectric semiconductorsadjacent to each other across the respective insulation layersalternately disposed; and dipping the thermoelectric device block havingsaid metallic film formed thereon in an electroless plating bath, andforming an electroless plating film on said metallic film and both endfaces of the respective thermoelectric semiconductors with the metallicfilm formed on the portion of the end face thereof.
 9. An electrolessplating method comprising the steps of: forming a metallic film made ofa metal on which an electroless plating film can be deposited on eitheran end face or the other end face of respective insulation layers,alternately, on the sides of both end faces of a thermoelectric deviceblock formed integrally with a plurality of thermoelectricsemiconductors, disposed with the respective insulation layersinterposed therebetween, such that the metallic film spans therespective insulation layers and a portion of respective end faces ofboth the thermoelectric semiconductors adjacent to each other across therespective insulation layers; and dipping the thermoelectric deviceblock having said metallic film formed thereon in an electroless platingbath, and forming an electroless plating film on said metallic film andboth end faces of the respective thermoelectric semiconductors with themetallic film formed on the portion of the end face and the other endface thereof.
 10. An electroless plating method according to claim 6,wherein use is made of said thermoelectric device block provided with anexposed outer sidewall face of respective thermoelectric semiconductorspositioned at opposite ends in the direction along which the respectivethermoelectric semiconductors are arranged, and an electroless platingfilm is also formed on the exposed outer sidewall faces of respectivethermoelectric semiconductors positioned at opposite ends as well insaid step of forming the electroless plating film.
 11. An electrolessplating method according to claim 6, further comprising the step ofrendering the end face of the thermoelectric device block into a roughsurface prior to the step of forming the electroless plating film onsaid thermoelectric device block.
 12. An electroless plating methodaccording to claim 6, further comprising the step of cleaning saidthermoelectric device block before or after the step of forming theelectroless plating film on said thermoelectric device block.
 13. Anelectroless plating method comprising the steps of: forming a metallicfilm made of a metal on which an electroless plating film can bedeposited on part of the surface of an object to be plated, or causingthe metal to be in contact with part of the surface of the object to beplated, made of a constituent material to which an electroless platingcan not be applied; dipping the object to be plated having the metallicfilm formed thereon or having the metal in contact therewith in anelectroless plating bath, and forming an electroless plating film on theentire surface of the object to be plated, containing the metallic filmor the metal, including the surface of the object to be plated withoutthe metallic film formed thereon and the metal in contact therewith;removing the metallic film or the metal, and portions of the electrolessplating film, covering up the metallic film or the metal, from theobject to be plated; and dipping again the object to be plated subjectedto the steps described above in the electroless plating bath, whereinthe object to be plated is made of plural kinds of constituentmaterials.
 14. An electroless plating method comprising the steps of:forming a metallic film made of a metal on which an electroless platingfilm can be deposited on part of the surface of an object to be plated,or causing the metal to be in contact with part of the surface of theobject to be plated, made of a constituent material to which anelectroless plating can not be applied; dipping the object to be platedhaving the metallic film formed thereon or having the metal in contacttherewith in an electroless plating bath, and forming an electrolessplating film on the entire surface of the object to be plated,containing the metallic film or the metal; removing the metallic film orthe metal, and portions of the electroless plating film, covering up themetallic film or the metal, from the object to be plated; and dippingagain the object to be plated subjected to the steps described above inthe electroless plating bath, wherein the object to be plated is athermoelectric semiconductor.
 15. An electroless plating methodcomprising the steps of: preparing an object to be plated, comprised ofmetal or semiconductors, to which an electroless plating can not beapplied, and insulators; forming a metallic film made of a metal onwhich an electroless plating film can be deposited on part of thesurface of the object to be plated, or causing the metal to be incontact with part of the surface of the object to be plated; dipping theobject to be plated having the metallic film formed thereon or havingthe metal in contact therewith in an electroless plating bath; andforming an electroless plating film on the entire surface of the objectto be plated, except for the insulators, including the surface of theobject to be plated without the metallic film formed thereon and themetal in contact therewith.
 16. An electroless plating method accordingto claim 1, wherein use is made of a metal or a semiconductor, to whichan electroless plating can not be applied, as the constituent materialto which an electroless plating can not be applied.
 17. An electrolessplating method according to claim 2, wherein use is made of a metal or asemiconductor, to which an electroless plating can not be applied, asthe constituent material to which an electroless plating can not beapplied.
 18. An electroless plating method according to claim 3, whereinuse is made of a metal or a semiconductor, to which an electrolessplating can not be applied, as the constituent material to which anelectroless plating can not be applied.
 19. An electroless platingmethod according to claim 13, wherein use is made of a metal or asemiconductor, to which an electroless plating can not be applied, asthe constituent material to which an electroless plating can not beapplied.
 20. An electroless plating method according to any one ofclaims 1 to 4, claims 6 to 9, and claims 13 to 15, wherein use is madeof palladium, platinum or nickel as the metal on which the electrolessplating film can be deposited.
 21. An electroless plating methodaccording to claim 15, wherein use is made of an insulating resin forthe insulators.
 22. An electroless plating method according to any oneof claims 6 to 9, wherein use is made of an insulating resin for theinsulation layers.